Viral phenomenon: Ancient microbe invaded human DNA

Humans carry in their genome the relics of an animal virus that infected their forerunners at least 40 million years ago, according to research published Wednesday by the British science journal Nature.

The invader is called bornavirus, a brain-infecting pathogen that was first identified in 1970s.

Scientists led by Keizo Tomonaga of Japan's Osaka University compared the DNA of a range of mammals, including humans, apes, elephants, marsupials and rodents, to look for tell-tale signatures of bornavirus code.

In the human genome, the team found several bornavirus fragments but also in the form of two genes that may be functional, although what they do is unclear.

Until now, the only viruses known to have been handed on in vertebrates were retroviruses, which work by hijacking cellular machinery in order to reproduce.

Retroviruses are effective in infiltrating the germline -- the DNA of reproductive cells, which means their sequence, or part of it, is handed on to ensuing generations.